I'm not a gun control advocate, but I do think that the theory as to why we haven't been invaded is because our citizens are so well armed is preposterous.

Imagine a well trained division of invading soldiers, expert in urban warfare tactics, encountering a group of civilians who happened to break out their deer rifles and 9mm "self-protection" pistols. At least 90% of whom have never fired at a human target, much less killed anyone. Somehow I don't think the battle would bode well for the citizens.

IMHO, the real reason we haven't been invaded has already been expressed above. It has much more to do with sheer logistics, and our MILITARY defense capabilities, than a bunch of regular folks unloadin' the old double barrel.